A frequency duplexed Asymmetric Digital Subscriber Line (ADSL) modem transmits in one frequency band and receives in a second, disjoint frequency band. In an “Annex A” mode, a Customer Premises Equipment (CPE) modem transmits from about 26 kilohertz (KHz) to 138 KHz in an upstream direction to a Central Office (CO) mode. The CO modem transmits from 138 KHz to 1104 KHz in a downstream direction to the CPE modem. Since the CPE transmits in a lower frequency band than the CPE receives, the CPE transmit circuit non-linearity and modulation of an upstream data signal on transmit carriers causes some frequency content of the CPE transmit signal to appear in the CPE receive band as echo signals. The echo signals acts as noise and an impediment to downstream data transmission.
Echo signals are conventionally reduced in several ways. First, the transmitted signal from the CPE modem is filtered to reduce energy in the frequencies that cause echo. Second, an analog circuit called an “echo cancelling hybrid”, or “hybrid” for short, is used to measure and cancel the transmitted signal from the received signal. Finally, complicated adaptive techniques called echo cancellation can be used to suppress the echo further.
Some filtering will always be done. However, increasing the filtering degrades the upstream data rate because the filter will increase the phase loss in the pass band, making the upstream channel more difficult for the CO modem to equalize. Additional filtering also requires more electronic components, which can increase the manufacturing cost of the modems. Likewise, the hybrid circuit will always be used. However, because of the variation of phone lines, a non-programmable hybrid circuit design must sacrifice echo signal attenuation for robustness. Conversely, a hybrid optimized for a particular phone line without bridged taps may perform unacceptably for phone lines with moderate length bridged taps. Furthermore, required digital and analog hardware support used to implement the echo cancellation functions add to an expense and design complication for the modems.